The Institute of Electrical and Electronics Engineers (IEEE) has developed several 802.11X specifications that define communication protocols used by network devices operating in wireless local area networks (WLANs). For example, the communication protocols include authentication schemes that can be used to securely exchange data between the network devices. The communication protocols include power-saving strategies that can be used to save power in the network devices. The communication protocols include synchronization schemes to synchronize clocks of the network devices, and so on. Some protocols include a fine timing measurement (FTM) procedure used to measure the time of flight (ToF) of a radio frequency (RF) signal between two network devices. The ToF is used to measure a range (i.e., distance) between the network devices.
FIG. 1 shows an example FTM procedure used to measure ToF between two network devices—client stations STA1 and STA2—operating in a WLAN. STA2 initiates an FTM request by sending a request frame (FTM request) to STA1. STA1 receives the FTM request at time t0 and responds by sending a corresponding acknowledgment frame (ACK). The FTM request and the corresponding acknowledgement frame are followed by STA1 sending an FTM response frame (FTM_1) to STA2. The FTM response frame leaves a transmit antenna of STA1 at time t1 and arrives at a receive antenna of STA2 at time t2. STA1 and STA2 may measure timestamps indicative of when the FTM response frame leaves the transmit antenna from STA1 (t1) and when the FTM response frame arrives at the receive antenna at STA2 (t2). STA2 responds by sending an acknowledgement frame to STA1. The ACK frame leaves a transmit antenna of STA2 at time t3 and arrives at a receive antenna of STA1 at time t4. STA2 and STA1 measure timestamps indicative of when the ACK frame leaves the transmit antenna of STA2 (t3) and when the ACK frame arrives at the receive antenna of STA1 (t4).
The FTM response frame includes several parameters that describe the range measurement bursts, including a burst offset field (i.e., a field that indicates a value of a burst offset). STA2 transmits the FTM request to trigger the first FTM burst at a time indicated by the burst offset field of the FTM Response. The burst offset corresponds to a duration (e.g., 10 ms) between reception of the FTM request by STA1 and a start of the first burst period. Accordingly, the burst offset provides an indication, to the stations STA1 and STA2, of the start of the first burst period. The stations perform the FTM based on frames transmitted during the burst period. The burst period corresponds to a duration between a start of one burst period (e.g., burst period 1) to a start of a next burst period (e.g., burst period 2).
For example, STA2 transmits another FTM request to STA1 at the start of the burst period 1 to notify STA1 that STA2 is ready to receive FTM frames for range measurement. STA1 transmits an ACK and then provides t1 and t4 to STA2 in an FTM (FTM_2). STA2 calculates a round trip time (RTT), which is twice the ToF between STA1 and STA2, as RTT=(t4−t1)−(t3−t2)=(t2−t1)+(t4−t3). The ToF between STA1 and STA2 is RTT/2. STA2 can perform additional calculations based on transmitted frames and respective times in burst period 2.